To come up with an idea that you aren't aware is already around. From "retro" meaning backwards/before and "innovate".

To retrovate something, one must think it's an entirely new idea. That is, he must not have any prior knowledge of the idea being around. (Retrovation is a form of plagiarism if the idea is published.)

Man: "I had an awesome idea. You could go make me a sandwich, woman!"
Woman: "You might've just retrovated that, honey."
Man: "No back-talk woman, make me a sandwich!"

Guy: "I coined the phrase nbd two years ago, now everyone I know is using it!"
Girl: "Wow, that's pretty badass. . . Except it was the urbandictionary.com word of the day July 2, 2007. I think you might've retrovated it."
Guy: "God dammit... Sleep with me anyway?"
Girl: "Wasn't going to happen to begin with."

lamer21: "Dude, there was an xkcd on retrovation, check it out, it's comic 827."
blackHatDude: "Retrovation means you had the idea after, it doesn't mean you had it before and never publicized it. By the way, I thought I should inform you, I hit your cat the other day, he didn't run away."